In the production of organic saturated aliphatic monocarboxylic acids having 6 to 9 carbon atoms by oxidation of the corresponding aldehydes, soluble manganese and copper compounds, such as manganous acetate and cupric acetate, are used in combination as catalysts to provide high carbon conversion and high efficiency of aldehyde to acid. High carbon conversion and high efficiency essential not only to provide good yields of acids but also to produce relatively small amounts of undesirable by-products and thus avoid recycling unreacted starting materials. The oxidation reaction can be conducted in the liquid phase using a single stage or multiple stage reactors. These reactions can be operated under pressure in the range from about 60 to about 150 pounds per square inch gauge, preferably from about 85 to about 95 pounds per square inch gauge, with air or oxygen containing gas in the temperature range from about 50.degree. C. to about 80.degree. C. Such a process is described in copending application U.S. Ser. No. 065,241 filed Aug. 9, 1979 assigned to the same assignee and filed concurrently with this application.
However, it has been found that metallic copper will precipitate from acids prepared using such mixed soluble catalyst compounds due to reduction during the processing of these acids, especially during distillation. The presence of precipitated copper particles in turn can lead to serious mechanical problems such as reboiler fouling and erosion of pump impellers. Manganese compounds, on the other hand, tend to remain soluble and are not readily reduced to the metal.
Removing the soluble metal catalysts from the acid products would not only avoid the aforementioned mechanical problems but would also provide purer acid products. Copending application U.S. Ser. No. 065,239 filed Aug. 9, 1979, assigned to the same assignee and filed concurrently with this application, describes the precipitation of copper and manganese as oxalates from 3-9 carbon acid products containing them by the addition of oxalic acid thereto. The copper and manganese oxalates can then be filtered from the acid products by normal filtration, using a continuous centrifuge or other separation techniques.
In the prior art, there are various techniques to describe the removal of metal catalysts from the reaction product. In U.S. Pat. No. 3,840,469, there is a disclosure for the cobalt catalyst recovery from an acetic acid medium derived from the liquid phase oxidation of aliphatic hydrocarbons. This procedure precipitates the cobalt as cobalt oxalate in the acetic acid product. The patent indicates that manganese would not undergo precipitation in this procedure. In U.S. Pat. No. 2,380,731, a procedure is described using oxalic acid to remove numerous metals such as iron, magnesium, chromium, copper, vanadium etc. from a refractory inorganic support such as a clay or alumina or silica or an alumina-silica refractory catalytic cracking catalyst. These procedures are not related to the process of this invention.
It is the purpose of this invention to provide an improved procedure for separating soluble manganese and copper catalysts from acid products containing them.